A light emitting diode (LED) is a photoelectric conversion semiconductor device in which an N type semiconductor and a P type semiconductor are joined together, and emits light through recombination of electrons and holes.
Light emitting diodes include a red LED using GaAsP or the like, a green LED using GaP or the like, a blue LED using an InGaN/AlGaN double hetero structure, and the like.
This light emitting diode is packaged and then used in manufacturing a variety of light emitting devices. The light emitting device manufactured from the packaged light emitting diodes has characteristics of low power consumption, a long lifespan, installation in a narrow space, and strong resistance against vibration. In recent years, white LEDs in addition to single color LEDs, e.g. red, blue or green LEDs, have been placed on the market. As the white LEDs are applied to products for automobiles and illumination, it is expected that their demands will be rapidly increased.
In light emitting diode technologies, the methods of implementing white color can be roughly classified into two types. The first one is a method in which red, blue and green LEDs are arranged to be adjacent to one another and colors of light emitted from the respective devices are mixed to implement white light. However, since the respective light emitting diodes have different thermal or temporal characteristics, there are problems in that uniform light mixing cannot be obtained due to changes in a color tone according to usage environment, particularly, the occurrence of color spots, or the like, and thus, the brightness is not sufficiently high. Further, the circuit configurations for operating the respective light emitting diodes are complex, and it is difficult to implement perfect white light since it is difficult to obtain optimal conditions for mixing three color lights depending on the positions of the light emitting diodes due to the package configurations. Moreover, since its Color Rendering Index (CRI) is as low as about 40, it is not suitable for the general lighting source or the flash.
The second one is a method in which a phosphor is disposed on a light emitting diode, and the color of a portion of primary light emitted from the light emitting diode and the color of secondary light of which wavelength has been converted by the phosphor are mixed to implement white light. For example, to a light emitting diode for emitting blue light is attached a phosphor that emits yellowish green or yellow using a portion of the blue light of the light emitting diode as an excitation source, so that white light can be obtained by mixing the blue light emitted from the light emitting diode and the yellowish green or yellow light emitted from the phosphor. Alternatively, on a light emitting diode for emitting ultraviolet rays may be applied a phosphor that absorbs the emitted ultraviolet rays and emits visible light ranging from blue to red in order to obtain white color.
In order to obtain white light emission, the blue LED with a wavelength of 450 to 470 nm and the yellow phosphor such as YAG:Ce or (Ba,Sr,Ca)2SiO4;Eu may be generally used. The blue light source causes the yellow phosphor to be excited and then to emit yellow light and, and thus, white light can be obtained by mixing the blue and yellow lights.